FORT WORTH, Texas − Elephants in zoos are living as long as those in the wild, but their population in captivity is dwindling because it is so difficult to breed them, according to studies released Friday.

The study of elephant life spans concluded that the average life expectancy of Asian and African elephants in captivity in North America is 45 years and 33 years, respectively. Those figures are similar for elephants in the wild, according to the study, presented Friday at the International Elephant Foundation's research symposium and published recently in Zoo Biology magazine.

Earlier studies that said captive elephants lived about 20 years on average used data only from animals that had died, said Bob Wiese, the Fort Worth Zoo's director of animal collections.

His study examined dead and living elephants, including a 77-year-old Asian and a 53-year-old African elephant in North America.

RaeLeann Smith, a spokeswoman with the animal advocacy group In Defense of Animals, accused zoos of "playing with numbers." She said elephants in the wild do not suffer from the reproductive, digestive and foot and joint problems that afflict animals in captivity.

"Our concern is that elephants are highly intelligent, social animals and zoos are unable to meet their physical and psychological needs," Smith said.

African elephants, which are threatened in the wild, weigh up to 7 tons and are up to 14 feet tall. Asian elephants, which are endangered in the wild, weigh 3 tons to 6 tons and are up to 12 feet tall.

There are up to 30,000 Asian and half a million African elephants in the wild, according to the foundation. In captivity worldwide, there are up to 16,000 Asian -- including as many as 300 in North America -- and up to 700 African elephants -- including 230 in North America.

Breeding elephants in captivity has proven difficult.

Last year, the Fort Worth zoo eagerly awaited a calf's birth to a 30-year-old Asian elephant, but the pregnant animal went into labor six months too soon. The calf was euthanized.

There have been three successful Asian elephant births so far this year in zoos accredited by the American Zoo and Aquarium Association, but there were none in the previous two years.